The present invention relates to a toner for developing an electrostatic latent image formed on a latent-image carrier and to an image forming apparatus for forming an image using this toner.
More particularly, the present invention relates to a non-magnetic mono-component toner to be used for developing an image according to a non-magnetic mono-component developing method using a conductive developing roller and to an image forming apparatus using the non-magnetic mono-component toner.
Further, the present invention relates to a toner in which additive particles are entrapped in mother particles and to an image forming apparatus using this toner.
Among conventional known image forming apparatuses, there is a multicolor image forming apparatus of intermediate transfer type as shown in FIG. 1. In an image forming apparatus 1 of this type, an image is exposed to light as an electrostatic latent image onto a photoreceptor 2 as a latent image carrier. The electrostatic latent image on the photoreceptor 2 is developed by yellow, magenta, cyan, and black non-magnetic mono-component developing devices 3, 4, 5, 6 in this order (the order of respective colors is arbitrary) so as to obtain visible developed images. The developed images on the photoreceptor are superposed and toned on an intermediate transfer belt 7a of an intermediate transfer member 7, thus achieving primary transfer. After the primary transfer, the toned image is transferred to a recording media 9 such as a paper on a secondary transfer roller 8a of the transferring device 8, thus achieving secondary transfer. After that, the image is heated and fixed to the recording media 9 by a fixing device 10, thereby obtaining a desired image on the recording media 9.
The respective non-magnetic mono-component developing devices 3, 4, 5, 6 have substantially the same structure. That is, each of the developing devices 3, 4, 5, 6 is of a contact developing type that a conductive developing roller 16 is arranged in contact with the photoreceptor 2. As shown in FIG. 2(a), non-magnetic mono-component toner particles T in a toner container 13 are carried by a toner carrying means 14 to a toner supply roller 15 as toner supply means and is further supplied to the developing roller 16 by the toner supply roller 15. Accordingly, the non-magnetic mono-component toner particles T are held on the surface of the developing roller 16. A bias voltage composed of an alternating current superimposed on a direct current is applied to the developing roller 16 and the developing roller 16 is rotated at a high speed, whereby the non-magnetic mono-component toner particles T are regulated to be in a uniform thin layer by a toner regulating member 17 as toner regulating means, which is in press contact with the surface of the developing roller 16, and are uniformly charged. After that, the non-magnetic mono-component toner particles T on the developing roller 16 are uniformly conveyed toward the photoreceptor 2 which is in contact with the developing roller 16.
The non-magnetic mono-component toner particles T on the developing roller 16 is transferred to the photoreceptor 2 by developing voltage applied to the developing roller 16, whereby an electrostatic latent image is developed with the non-magnetic mono-component toner particles T on the photoreceptor 2.
The non-magnetic mono-component developing method employing conductive developing roller 16 also includes a non-contact developing method in which the developing roller 16 and a photoreceptor 2 are spaced apart from each other. In developing of the non-contact developing method, a developing voltage is applied to the developing roller 16 and non-magnetic mono-component toner particles T on the developing roller 16 is transferred by jumping to the photoreceptor 2 due to the developing voltage. That is, an electrostatic latent image on the photoreceptor 2 is developed with the non-magnetic mono-component toner particles T by jumping-developing.
By the way, in the non-magnetic mono-component developing method using such a conductive developing roller 16, both cases of the non-contact development or contact development, an image force Fm acts between the non-magnetic mono-component toner particles T and the developing roller 16 as shown in FIG. 3. To securely separate the non-magnetic mono-component toner particles T from the developing roller 16 and to transfer the toner particles T to the photoreceptor 2, a coulomb force Fc capable of overcoming the image force Fm should be imparted to the non-mono-component toner particles T. Since the coulomb force Fc imparted to the non-magnetic mono-component toner particles T increases as the image force Fm increases, the developing voltage should be accordingly increased. However, too large developing voltage must cause discharge. Therefore, the developing voltage can not be increased so large. This means that the developing bias has an upper limitation. There is accordingly a problem of narrow margin for developing.
As the image force Fm is large, the effect of removing residual toner particles remaining on the developing roller 16 after developing by the toner supply roller 15 is reduced, thus facilitating producing undesirable developing hysteresis. That is, when the image force Fm is not so large, residual toner particles remaining on the developing roller 16 can be peeled off by the toner supply roller 15 as shown in FIG. 4(a). Therefore, the residual toner particles do not pass through spaces between the developing roller 16 and the toner supply roller 15 and thus does not move to the toner regulating means 17 side. Therefore, toner particles on the developing roller 16 between the toner supply roller 15 and the toner regulating means 17 are new non-magnetic mono-component toner particles T supplied from the toner supply roller 15 only.
On the other hand, when the image force Fm is so large, the residual toner particles can not be completely peeled off by the toner supply roller 15 as shown in FIG. 4(b). Therefore, some of the residual toner particles pass through spaces between the developing roller 16 and the toner supply roller 15 and thus move to the toner regulating means 17 side. Since no non-magnetic mono-component toner particle T is newly supplied from the toner supply roller 15 to portions, to which residual toner particles adhere, of the developing roller 16, the residual toner particles are carried as developer and are further charged by passing through the toner regulating means 17 so as to have larger charge. Difference in charge leads to difference in image density between a portion developed with the residual toner particles and a portion developed with new non-magnetic mono-component toner particles. That is, undesirable developing hysteresis appears.
After a solid image is printed, the amount of residual toner particles is reduced and the influence of the image force Fm is thus reduced, so none of the residual toner particles passes through spaces between the developing roller 16 and the toner supply roller 15. Therefore, non-charged new non-magnetic mono-component toner particles T, which are not charged yet, are supplied to the developing roller 16 from the toner supply roller 15 so that the non-magnetic mono-component toner particles T are suitably charged by the toner regulating means 17. However, after a white solid image is printed, the amount of residual toner particles is increased because most of the non-magnetic mono-component toner particles T are not developed, so the influence of the image force Fm is thus increased. As the image force Fm is large, some of the residual toner particles pass through spaces between the developing roller 16 and the toner supply roller 15. The residual toner particles are further charged by passing through the toner regulating means 17 so as to have larger charge. The charge of non-magnetic mono-component toner particles differs due to what type of image printed by just the last circle of the developing roller 16, particularly, solid image or white solid image. Therefore, even when printing the same pattern, density unevenness appears. That is, undesirable developing hysteresis appears.
On the other hand, a toner T conventionally used contains additive particles which are dispersed in mother particles in order to improve the characteristics of the toner or to facilitate the preparation of the toner.
For example, to stabilize the electric characteristic of the toner, a charge controlling agent called as “CCA” is dispersed as an additive in mother particles. To impart manifestation of color and/or permeability to the toner, a pigment is entrapped and dispersed as an additive in mother particles.
Further, the conventional toners are prepared by a pulverization method or polymerization method. In toners prepared by pulverization, a mold releasing agent and/or a pulverization assisting agent are kneaded with mother particles of the toner. In toners prepared by polymerization, a polymerization reaction assisting agent such as an initiator or dispersant is used as an additive for polymerization reaction.
However, some particles of such additives entrapped in mother particles of the toner may be liberated from the mother particles to become liberated additive particles. The liberated additive particles may affect the carrying property and charging property of the toner because the liberated additive particles adhere to the surface of a processing member such as the developing roller of the developing device.
For example, some particles of a CCA do not enter into mother particles during preparation of the toner so as to form liberated CCA particles liberated from the mother particles. On the other hand, the surface of the developing roller may be exposed at locations where a large amount of toner particles is consumed. As the surface of the developing roller is exposed, when new toner is supplied to the developing roller, the liberated CCA particles in the new toner adhere to the exposed surface of the developing roller. Since the liberated CCA particles have a particle diameter smaller than that of the mother particles and have a charging property stronger than that of the mother particles, once the liberated CCA particles adhere to the surface of the developing roller, the CCA particles are hardly separated from the developing roller because of image forces and intermolecular forces therebetween. Moreover, the adhering force between the liberated CCA particles and the developing roller is increased by friction created by the toner regulating member and the like.
As many liberated CCA particles adhere to the developing roller, there are differences in carrying characteristic and charging characteristic of the toner between a portion where many liberated CCA particles adhere and a portion where little liberated CCA particles adhere. There are differences in amount of toner developed on the photoreceptor between the aforementioned portions. As a result, a band-shaped unevenness of density appears on the resultant image. The adherence of the liberated CCA also shortens the lifetime of the developing device.
Therefore, it is desired to prevent liberated CCA particles from adhering to processing members such as the developing roller.
As another example, parts of pigment do not enter into mother particles during preparation of the toner so as to form pigment liberated from the mother particles. A portion of the resultant image corresponding to portion where the liberated pigment adheres to a processing member such as the developing roller should be white blank.
Further, the liberated pigment particles have a diameter larger than the pigment particles dispersed in the mother particles. Pigment functions “to impart manifestation of color” and “to impart permeability to a toner” when dispersed as micro particles in mother particles of resin. When the ratio of liberated pigment relative to entire pigment is high, the manifestation of color and the permeability may be affected. That is, poor manifestation of color leads to insufficient density and poor permeability leads to poor permeability of OHP.
Therefore, it is desired to prevent liberated pigment from adhering to the developing roller and other processing members.
Further in toners T prepared by pulverization, metallic soap is frequently kneaded as additives such as a mold releasing agent and a pulverization assisting agent together. Poor dispersion of the metallic soap leads to liberation of metallic soap particles from the mother particles. As the liberated metallic soap particles adhere to a processing member such as the developing roller, image defects such as image blurs may be produced.
Therefore, it is desired to prevent liberated metallic soap particles from adhering to processing members such as the developing roller.
Furthermore, in toners prepared by polymerization such as emulsion polymerization, a polymerization reaction assisting agent as an additive such as an initiator or dispersant may not be completely used in polymerization reaction, so a part of the polymerization reaction assisting agent not used becomes residue after dehydrating and drying processes. The residual polymerization reaction assisting agent may exist in the state liberated from the mother particles in the toner particles. The liberated polymerization reaction assisting agent facilitates coagulation of toner and thus provides poor fluidity of toner. Therefore, a predetermined carrying rate may not be obtained.
Therefore, it is desired to improve the fluidity of polymerized toner.